1. Field of the Invention
The present invention relates generally to the field of video-tape-recorder-design based anti-copying processes for video systems, and more particularly to the application of those processes to digital video recording systems.
2. Related Art
Televisions utilize cathode ray tubes (CRTs) to display video images. CRTs have electron guns that produce an electron beam. The beam is attracted to phosphors on the face of the CRT, activating the phosphors and causing them to emit light. The electron beam begins at the top left of the CRT and scans from left to right across the screen, illuminating pixels (which are comprised of the activated phosphors) in the process. Hence, the electron beam effectively draws horizontal lines of video, one pixel at a time.
The horizontal scan rate is the number of horizontal lines drawn per second by the electron beam generated by the electron gun in a video display. Television broadcast standards specify exact horizontal scan rates that must be strictly adhered to in television video signals. The National Television Standards Committee (NTSC) standards are used in North America and other parts of the world. The Phase Alteration Line/Sequential Couleur Avec Mémoire (PAL/SECAM, or PAL) standards are used in Europe and elsewhere.
The NTSC horizontal scan rate is 15.75 kHz and the PAL horizontal scan rate is 15.625 kHz. When the electron beam reaches the bottom of the display, one frame of video has been completed. The number of frames completed by the beam per second, or the number of times that the frame has been “refreshed”, is the vertical scan, frame or “refresh” rate.
A television display uses an “interlaced” scanning format. Each frame of video is scanned out as two fields that are separated temporally and offset spatially in the vertical direction. Each field is drawn on the screen consecutively and in alternating fashion—first one field, then the other. Essentially, an image is drawn in two top-to-bottom passes: the first pass draws the “odd” lines (the first field) and the second pass draws the “even” lines (the second field). It follows that the number of lines in a field is one-half the number of lines in a frame. In NTSC, there are 262.5 lines per field (525 lines per frame), and in PAL, there are 312.5 lines per field (625 lines per frame). An interlaced scanning format is utilized in television video signals because of the relatively slow frame rate of a television. Interlacing the scan lines avoids “flicker” in the image in a manner well understood in the art. Under the television broadcast standards, there are exact field rates that must be strictly adhered to: 59.94 Hz for NTSC and 50 Hz for PAL.
Television signals comprise a composite waveform that contains a number of specifically placed and timed video and control signals. These include the active video signal; the color burst waveform; the horizontal and vertical sync pulses; and the horizontal and vertical blanking intervals. The active video signal contains encoded luminance and chrominance data for the image that is to be displayed on the screen. The color burst waveform provides a decoder with a reference for decoding the chrominance information contained in the active video signal. The horizontal and vertical sync pulses are control signals that signal to the decoder the start of new horizontal lines and new frames. The blanking intervals signal the decoder to shut off the electron beam while it is being retraced from the right edge to the left edge of the display, or from the bottom to the top of the display. Each of these signals is combined into one composite video waveform that is transmitted to the television on a one-wire connection.
The composite video waveform must be encoded in strict accordance with the applicable broadcast standard, such as NTSC or PAL. These standards specify important timing parameters such as the horizontal and vertical sync pulse widths, the rise and fall times of the pulses, and the position and number of cycles in the color burst. These timing parameters should generally not be substantially altered while encoding the waveform. Numerous problems can result from even slightly inaccurate timing. Errors in the pulse widths can lead to picture break up, and errors in the rise and fall times can make it difficult for the television receiving equipment to lock to the signals. However, these timing parameters can be altered to gain anti-copying benefits.
The creators of video content for distribution have always been concerned with protecting their copyright in the works they produce. With the development of video cassette recorders (VCRs) and, more recently, digital video recording systems, this concern has grown substantially in recent years. Copyright holders have profited significantly from the development and proliferation of video recording systems. However, this same technology now allows a single inexperienced person with a single copy of a video program to create numerous unauthorized copies of a copyrighted work, thereby substantially reducing the legitimate demand for a particular work. Without anti-copying systems in place, copyright holders run the risk of losing control of their works with a single broadcast or release on videocassette.
To aid copyright holders in maintaining their rightful control of their copyrighted works, video-tape-recorder-design based anti-copying processes have been developed. These processes take advantage of the timing parameters built into broadcast standards, such as NTSC and PAL, and the design differences between televisions and traditional VCRs. A prominent example is Macrovision's anti-taping process. An early version of Macrovision's anti-taping process is disclosed and described in U.S. Pat. No. 4,631,603, “Method and Apparatus for Processing a Video Signal so as to Prohibit the Making of Acceptable Video Tape Recordings Thereof.” Further information regarding this process is available from Macrovision Corporation, 1341 Orleans Drive, Sunnyvale, Calif. 94089.
Anti-taping processes, such as Macrovision's process, utilize differences in the way traditional VCRs and televisions operate. Both VCRs and televisions have automatic gain control (AGC) circuits within them. However, the AGC circuits in typical VCRs are designed to respond quickly to change, whereas the AGC circuits in typical televisions respond to change slowly. Macrovision's anti-taping process modifies a television video signal so that a VCR will not record a viewable picture, but a television will continue to display the video signal properly.
Macrovision's process applies special waveforms to the video signal, which can be a broadcast signal, a VCR tape signal or the like. These waveforms prevent copying of the original by adding artifacts which have little effect on electronic devices, such as televisions, but which tend to confuse mechanical recording devices, such as VCRs. These artifacts include abrupt phase transitions added to the color strip, pseudo-sync pulses and AGC pulses added to predetermined scan lines in the vertical blanking interval (VBI), and additional AGC pulses added to the “back porch” of the video signal. The “back porch” is the portion of the video waveform that extends from the rising edge of a horizontal sync pulse to the beginning of the active video signal.
The pseudo-sync pulses are added to the predetermined scan lines in the VBI to fool the sync detection circuits of a recording VCR into believing a horizontal sync is occurring. Immediately after each such pseudo-sync pulse, an AGC pulse is added. The AGC pulses continuously vary in amplitude, and thereby fool the AGC circuits of the VCR into believing the blanking level is incorrect. Thus these circuits will attempt to adjust the gain, when no such adjustment is needed, thereby leading to the creation of generally unviewable pictures.
The problem with this type of anti-taping process is that current video recording technology is moving away from the traditional mechanical devices, exemplified by the typical VCR. For example, television video capture cards are electronic circuit boards, which can be coupled with a personal computer to enable a computer user to capture single frame images or whole video segments from a television video signal. The false pulses that normally throw off the time tracking of a VCR have little effect on the timing of the electronic hardware making up modern digital video recording systems.
Analog television signals, protected by video-tape-recorder-design based anti-copying processes, can be captured by digital video recording systems and saved in memory as digital video, thereby enabling easy copying of copyrighted video programs. In addition, modern digital VCRs can record analog television signals, protected by video-tape-recorder-design based anti-copying processes, without being fooled by the artifacts added to the signal. Thus, as recording systems move away from mechanical devices, such as traditional VCRs, to digital video recording systems, traditional video-tape-recorder-design based anti-copying processes become ineffective. Eventually, video-tape-recorder-design based anti-copying processes, such as Macrovision's process, will become obsolete unless effective methods are found to apply such anti-copying processes to modern digital video recording systems.
Therefore, what is needed is a system and method for applying traditional video-tape-recorder-design based anti-copying processes to modern digital video recording systems.